1. Field of the Invention
This invention relates to a method and apparatus for embedding drain boards, which can expand the diameter of an effective drain circle to achieve the maximum improvable effect with a relatively small number of embodiments in submarine foundations, coastal reclaimed land or the like needing improvements on foundations of the depths.
2. Description of the Prior Art
Conventionally, as shown in FIG. 1, a general drain board used for improving foundations is a strip-like body having a section of about several mm thickness and 10 cm width and constituted from a base plate 3 having on both surfaces a continuous groove 2 and non-woven fabrics 4,4 having water permeability and secured to the surfaces of the base plate 3.
Also, since the time required for embedding and withdrawing such drain boards to be used for draining deep submarine foundations must be greater than that for improving shallow foundations, a concept of expanding the diameter of the effective drain circle by substantially enlarging the width of the board has been developed.
The effective drain circle means the size of absorption and drain range in which the drain board driven into the ground can take charge of and which is represented by the section as shape centering around the board. Since a general sand drain or the like itself has a circular section, said circle has a circular section. However, since the drain board is plate-shaped, it is found from the measurement of isobaric level of absorption and drain as shown in FIG. 2 that said circle becomes actually plate-shaped at a position near the board, gradually elliptic as it is spaced from the board, and approximately circular as it is further spaced from the board.
This is caused by the directional properties of absorption and drainage inherent in the board itself and such properties increase as the width of the board is increased. That is, as shown in FIG. 2, while the isobaric lines E of absorption and drainage of the drain board 1 resemble a circle as they are spaced farther from the board, the absorption and drainage potential is higher along the isobaric lines closer to the board. Thus, the directional properties of this plate-shaped circle present a problem which cannot be neglected, particularly in using a board of large width.
Thus, the maximum width of the board which has been heretofore considered optimum for embedment has been 20 cm-25 cm. With the width exceeding 20 cm-25 cm, the effective drain circles of drain boards embedded at equal intervals into the foundation do not properly overlap each other, with the consequence that irregular drainage may take place.
In addition to such a phenomenon, the portion of a mandrel for driving the large width board into a submarine foundation is plate-shaped like the drain board, so that from a structural point of view its strength is limited, as well as the board itself, which, because of lack of strength, should be limited to about 25 cm in width in order to avoid problems of twist, hanging-up, etc. A board having a width as large as 50 cm was considered to be out of the question.
In summary, since it was conventional wisdom to believe that the capacity of a drain board could be increased only by increasing the width of the drain board, the capacity of the drain board was inherently limited.